Tomographic imaging involves the acquisition of data that depict a map of some physical features along cross-sections through a volume in three-dimensional space. Tomography is a technique for generating images of a predetermined plane section of a solid object while blurring out the images of other planes. Several imaging modalities, most notably ultrasound imaging and magnetic resonance imaging (MRI), allow acquisition of such cross-sections in any desired orientation. This capability is highly desirable in a range of medical imaging applications, in which preferred image orientations exist for optimal diagnostic or other use of the image data. However, the preferred orientation and location of the image planes are in many cases not easily determined by analysis of external landmarks on the body to be imaged, and must be determined through feedback from initial images acquired within the study. The process of arriving at the desired scan geometry is in many applications a widely recognized difficult problem.
It is noted at this point that the present discussion is primarily directed to the prior art as it exists in the field of MRI, though it will be appreciated by those of ordinary skill in the art that other imaging modalities, such as ultrasound imaging, would benefit from the invention as described herein.
Scan geometry definition in MRI scanners is performed by encoding electronically controlled spatial components of pulsed electromagnetic fields in all three dimensions in such a manner that a desired spatial and temporal excitation pattern of nuclear magnetic spins is achieved. A sequence of radio-frequency signals, emitted in response by the nuclear spins, is captured by one or more receiver coils, digitized, and fed into a computer, where it is reconstructed into an image that reflects the spatial distribution of one or more features of the spin excitations. During this process, operator control over the geometry of the tomographic two-dimensional (2-D) plane or planes, of three-dimensional (3-D) volume, typically exists in modification of the associated parameters that govern the pulse sequence to reflect the desired geometry.
The size and the complexity of the parameter set prohibit in most instances control through direct modification of numerical values alone. Current MRI scanners provide geometry control through a visual interface by showing the operator the location of the anticipated scan in relation to images acquired earlier in the same study. To accommodate this, scanners typically allow the operator to rapidly acquire a limited number of initial images with predetermined suboptimal geometry at the beginning of the study. One or more of these localizer images (also referred to as scout images or reference images) may then be displayed on the computer screen, with intersection lines as overlay graphics depicting the anticipated location and orientation of the next scan while it is being defined. Either visual feedback of parameter changes is obtained by changes in position and orientation of these intersection lines, or the operator may actively interact with the intersection lines on the computer screen through pointer device manipulation, upon which the associated parameter changes are processed by the computer. A combination of these methods is the current state of the art.
Prior art generally defines a scan plane through intersecting lines on a 2-D tomographic localizer image display. The scan plane is defined through a line located on the 2-D image, and perpendicular to the displayed image, as shown in FIG. 1, which is discussed in more detail below. A shortcoming of this approach is that in cases with multiple non-orthogonal localizer images, the scan plane no longer can be defined perpendicular to the displayed images. In such situations the mental picture of the plane location tends to lose intuitiveness, and operator interactions to control the plane location in one image, through moving or rotating the definition lines, often have unexpected results in the other localizer planes. This can make plane definition a difficult iterative process. The proposed invention restores a desired level of intuition to this process.
A similar methodology is commonly employed in situations where a tomographic data volume has already been acquired with high resolution in three dimensions, but in a view plane orientation not properly aligned with the anatomy of interest. This situation presents itself sometimes in MRI, and often in X-ray Computed Tomography (CT), a modality with much more limited options in acquisition plane orientation. In such cases the geometry should be defined for a re-slicing or reformatting of the data volume to achieve the desired viewing plane, instead of for actual data acquisition along a plane.
These and other limitations of the current state of the art thus result in an iterative and often non-intuitive and confusing process to arrive at double-oblique image planes whenever these planes are required. Double-oblique planes are defined in this context as image planes that have no in-plane directions in common with the reference views. Single-oblique views, that have one common axis with a scout image, are completely defined by the common axis, displayed as an intersection line in the reference image, and its other axis, known to be perpendicular to the reference plane. Such a situation, as illustrated in FIG. 1, provides a certain level of intuitive understanding to the observer of these views. In the case of double-oblique intersections, it is no longer possible to assume that the plane represented by the intersection line is always orthogonal to the viewing plane. In such case, the observer must rely on simultaneous viewing of multiple reference views to mentally construct the geometry, also understanding and taking into account the geometrical relation between the reference views themselves. Interaction with the geometry definition by moving intersection lines in one reference view brings about changes in the intersection of the new plane geometry in other reference views that are often undesired and difficult to understand. Such undesired changes are then compensated by moving the intersection back to its intended location or orientation in the other reference view, possibly resulting in a confusing and time-consuming iterative process that does not always converge to the desired geometry.
As will be appreciated by those of skill in the art, another shortcoming of the prior art is in its failure to deal appropriately with cases where misregistration occurs between subsequently acquired localizer scans. In particular, this problem has been observed in cardiac MRI scans, and may be associated with many possible sources of patient motion that may cause such misregistration. Side-by-side display of localizer images with intersection line graphics depicting the geometry of a desired scan geometry does not provide any information that helps the operator detect spatial misregistration between these localizer images. As a result, efforts to align the operator defined scan geometry with all desired anatomical landmarks in the localizer scans may fail, or, even worse, be perceived as successful but result in acquisitions in the wrong location or orientation.
Thus, there exists an unsatisfied need in the industry for visualization technology able to alleviate these and other problems in 3-D visualization.